1. Technical Field
The present invention relates, in general, to a switching mode power supply (SMPS) unit having a protection circuit for a brown-out condition and, more particulary, to a switching mode power supply apparatus having a protection circuit for a brown-out condition in which, if an input alternating voltage is below a predetermined value, it is detected so that a switching mode power supply apparatus cannot be actuated or, if actuated, its operation is stopped. As a result, the switching mode power supply apparatus and a load circuit connected to an output terminal of the switching mode power supply apparatus are protected from the influx of excessive current.
2. Related Art
Generally, electric or electronic equipment need a direct current power supply apparatus which converts an alternating current (AC) power source to a predetermined DC power source so as to enable the system therein to be operated, for which a switching mode power supply apparatus of small size and light weight is mainly employed as a DC power supply apparatus.
U.S. Pat. No. 4,999,730 presents a solution to this problem but with disadvantages in that a power control device is necessary to directly control the alternating input, the control circuit is complicated, and an auxiliary power source is necessary for the control circuit.
As described in more detail below, prior switching mode power supply apparatus have several disadvantages. A primary disadvantage occurs when an input alternating voltage is less than a normal value, that is, when a brown-out condition occurs. Under such circumstances, the feedback controller of the apparatus applies a control signal to a pulse width modulation (PWM) controller of the apparatus. As explained in more detail below, this causes current accumulating in a secondary coil of the transformer of the apparatus to increase by an amount corresponding to the enlarged duty cycle of the PWM pulse generated by the PWM controller. As a result, circuitry of the apparatus or the load circuit connected to the apparatus can malfunction or be damaged due to overcurrent. As a further result, equipment employing the apparatus can also be damaged or break down.